1. Field of the Invention
The present invention is directed to inhibitors of the uptake of monoamine neurotransmitters and pharmaceutically acceptable salts and prodrugs thereof, the chemical synthesis thereof, and the medical use of such compounds for the treatment and/or management of psychotropic disorders, anxiety disorder, generalized anxiety disorder, depression, post-traumatic stress disorder, obsessive-compulsive disorder, panic disorder, hot flashes, senile dementia, migraine, hepatopulmonary syndrome, chronic pain, nociceptive pain, neuropathic pain, painful diabetic retinopathy, bipolar depression, obstructive sleep apnea, psychiatric disorders, premenstrual dysphoric disorder, social phobia, social anxiety disorder, urinary incontinence, anorexia, bulimia nervosa, obesity, ischemia, head injury, calcium overload in brain cells, drug dependence, and/or premature ejaculation.
2. Description of the Related Art
In an attempt to breakdown or to help solubilize chemicals and nutrients that have been absorbed into the blood, the human body expresses various enzymes (e.g. the cytochrome P450 enzymes or CYPs, esterases, proteases, reductases, dehydrogenases, and the like) that react with the chemicals and nutrients to produce novel intermediates or metabolites. Some of the most common metabolic reactions of pharmaceutical compounds involve the oxidation of a carbon-hydrogen (C—H) bond to either a carbon-oxygen (C—O) or carbon-carbon (C—C) π-bond. The resultant metabolites may be stable or unstable under physiological conditions, and can have substantially different pharmacokinetic, pharmacodynamic, acute and long-term toxicity profiles relative to the parent compounds. For most drugs, such oxidations are generally rapid and ultimately lead to administration of multiple or high daily doses. There is therefore an obvious and immediate need for improvements of such drugs.
Chemical kinetics is the study of reaction rates. The activation energy Eact in chemistry is the energy that must be supplied to a system in order to initiate a particular chemical process. In other words, this is the minimum energy required for a specific chemical reaction to take place. A reaction will occur between two properly oriented molecules if they possess a minimum requisite energy. During the approach, the outer shell electrons of each molecule will induce repulsion. Overcoming this repulsion requires an input of energy (i.e. the activation energy), which results from the heat of the system; i.e. the translational, vibrational, and rotational energy of each molecule. If sufficient energy is available, the molecules may attain the proximity and orientation necessary to cause a rearrangement of bonds to form new substances.
The relationship between the activation energy and the rate of reaction may be quantified by the Arrhenius equation which states that the fraction of molecules that have enough energy to overcome an energy barrier—those with energy at least equal to the activation energy, Eact—depends exponentially on the ratio of the activation to thermal energy k=Ae−Eact/RT. In this equation, RT is the average amount of thermal energy that molecules possess at a certain temperature T, where R is the molar gas constant, k is the rate constant for the reaction and A (the frequency factor) is a constant specific to each reaction that depends on the probability that the molecules will collide with the correct orientation.
The transition state in a reaction is a short lived state (on the order of 10−14 sec) along the reaction pathway during which the original bonds have stretched to their limit. By definition, the activation energy Eact for a reaction is the energy required to reach the transition state of that reaction. Reactions that involve multiple steps will necessarily have a number of transition states, and in these instances, the activation energy for the reaction is equal to the energy difference between the reactants and the most unstable transition state. Once the transition state is reached, the molecules can either revert, thus reforming the original reactants, or the new bonds form giving rise to the products. This dichotomy is possible because both pathways, forward and reverse, result in the release of energy. A catalyst facilitates a reaction process by lowering the activation energy leading to a transition state. Enzymes are examples of biological catalysts that reduce the energy necessary to achieve a particular transition state.
A carbon-hydrogen bond is by nature a covalent chemical bond. Such a bond forms when two atoms of similar electronegativity share some of their valence electrons, thereby creating a force that holds the atoms together. This force or bond strength can be quantified and is expressed in units of energy, and as such, covalent bonds between various atoms can be classified according to how much energy must be applied to the bond in order to break the bond or separate the two atoms.
The bond strength is directly proportional to the absolute value of the ground-state vibrational energy of the bond. This vibrational energy, which is also known as the zero-point vibrational energy, depends on the mass of the atoms that form the bond. The absolute value of the zero-point vibrational energy increases as the mass of one or both of the atoms making the bond increases. Since deuterium (D) is two-fold more massive than hydrogen (H), it follows that a C-D bond is stronger than the corresponding C—H bond. Compounds with C-D bonds are frequently indefinitely stable in H2O, and have been widely used for isotopic studies. If a C—H bond is broken during a rate-determining step in a chemical reaction (i.e. the step with the highest transition state energy), then substituting a deuterium for that hydrogen will cause a decrease in the reaction rate and the process will slow down. This phenomenon is known as the Deuterium Kinetic Isotope Effect (DKIE) and can range from about 1 (no isotope effect) to very large numbers, such as 50 or more, meaning that the reaction can be fifty, or more, times slower when deuterium is substituted for hydrogen. High DKIE values may be due in part to a phenomenon known as tunneling, which is a consequence of the uncertainty principle. Tunneling is ascribed to the small size of a hydrogen atom, and occurs because transition states involving a proton can sometimes form in the absence of the required activation energy. A deuterium is larger and statistically has a much lower probability of undergoing this phenomenon. Substitution of tritium for hydrogen results in yet a stronger bond than deuterium and gives numerically larger isotope effects.
Discovered in 1932 by Urey, deuterium (D) is a stable and non-radioactive isotope of hydrogen. It was the first isotope to be separated from its element in pure form and is twice as massive as hydrogen, and makes up about 0.02% of the total mass of hydrogen (in this usage meaning all hydrogen isotopes) on earth. When two deuteriums bond with one oxygen, deuterium oxide (D2O or “heavy water”) is formed. D2O looks and tastes like H2O but it has different physical properties. It boils at 101.41° C. and freezes at 3.79° C. Its heat capacity, heat of fusion, heat of vaporization, and entropy are all higher than H2O. It is also more viscous and is not as powerful a solvent as H2O.
Tritium (T) is a radioactive isotope of hydrogen, used in research, fusion reactors, neutron generators and radiopharmaceuticals. Mixing tritium with a phosphor provides a continuous light source, a technique that is commonly used in wristwatches, compasses, rifle sights and exit signs. It was discovered by Rutherford, Oliphant and Harteck in 1934 and is produced naturally in the upper atmosphere when cosmic rays react with H2 molecules. Tritium is a hydrogen atom that has 2 neutrons in the nucleus and has an atomic weight close to 3. It occurs naturally in the environment in very low concentrations, most commonly found as T2O, a colorless and odorless liquid. Tritium decays slowly (half-life=12.3 years) and emits a low energy beta particle that cannot penetrate the outer layer of human skin. Internal exposure is the main hazard associated with this isotope, yet it must be ingested in large amounts to pose a significant health risk.
When pure D2O is given to rodents, it is readily absorbed and reaches an equilibrium level that is usually about eighty percent of the concentration that is consumed by the animals. The quantity of deuterium required to induce toxicity is extremely high. When 0 to as much as 15% of the body water has been replaced by D2O, animals are healthy but are unable to gain weight as fast as the control (untreated) group. Between 15 to 20% D2O, the animals become excitable. At 20 to 25%, the animals are so excitable that they go into frequent convulsions when stimulated. Skin lesions, ulcers on the paws and muzzles, and necrosis of the tails appear. The animals also become very aggressive; males becoming almost unmanageable. At 30%, the animals refuse to eat and become comatose. Their body weight drops sharply and their metabolic rates drop far below normal, with death occurring at 30 to 35% replacement. The effects are reversible unless more than thirty percent of the previous body weight has been lost due to D2O. Studies have also shown that the use of D2O can delay the growth of cancer cells and enhance the cytotoxicity of certain antineoplastic agents.
Deuteration of pharmaceuticals to improve pharmacokinetics (PK), pharmacodynamics (PD), and toxicity profiles, has been demonstrated previously with some classes of drugs. For example, DKIE was used to decrease the hepatotoxicity of halothane by presumably limiting the production of reactive species such as trifluoroacetyl chloride. However, this method may not be applicable to all drug classes. For example, deuterium incorporation can lead to metabolic switching which may even give rise to an oxidative intermediate with a faster off-rate from an activating Phase I enzyme (e.g. cytochrome P450 3A4). The concept of metabolic switching asserts that xenogens, when sequestered by Phase I enzymes, may bind transiently and re-bind in a variety of conformations prior to the chemical reaction (e.g. oxidation). This claim is supported by the relatively vast size of binding pockets in many Phase I enzymes and the promiscuous nature of many metabolic reactions. Metabolic switching can potentially lead to different proportions of known metabolites as well as altogether new metabolites. This new metabolic profile may impart more or less toxicity. Such pitfalls are non-obvious and have not been heretofore sufficiently predictable a priori for any drug class.
It has been hypothesized that the efficacy of venlafaxine (Effexor®) is mainly due to its ability to inhibit serotonin reuptake and, potentially, norepinephrine reuptake in neuronal cells. The latter is purported to take effect only at high doses. The drug substance is sold as a 50/50 racemic mixture of R- and S-enantiomers. The mechanism of action of this drug has been extensively studied.

The benefits and shortcomings of this drug have been extensively reviewed as well. Some of these shortcomings can be traced to metabolism-related phenomena. Venlafaxine is converted in vivo by oxidative and conjugative degradation to multiple metabolites, at least 48 of which are documented. The major metabolites include much phase I metabolism leading to demethylation at the oxygen and/or nitrogen centers, and cyclohexyl ring hydroxylation, as well as significant phase II metabolism including glucuronidation of the hydroxylated metabolites. Because this drug is metabolized by polymorphically-expressed isozymes of cytochrome P450 including CYPs 2C19 and 2D6, and because it can act as an inhibitor of CYP2D6, its application in polypharmacy is necessarily complex and has potential for adverse events. These CYPs are involved in the metabolism of many medications that are typically prescribed concurrently with venlafaxine. This phenomenon increases inter-patient variability in response to polypharmacy. An example of the critical need for improvement is the published interpatient variability observed in “poor metabolizers” having either defective CYP2D6 alleles or total lack of CYP2D6 expression. These patients fail to convert venlafaxine to its equipotent metabolite, O-desmethylvenlafaxine. Venlafaxine also suffers from a short half-life relative to the majority of serotonin reuptake inhibitors. The half-life of venlafaxine in humans is ˜5 hours, while its active metabolite has a T1/2 of ˜11 hours. As a consequence of its 5-11 hour pharmacological half-life, those taking venlafaxine are at significant risk of SRI discontinuation symptoms if the drug is abruptly discontinued. Furthermore, in order to overcome its short half-life, the drug must be taken 2 (BID) or 3 (TID) times a day, which increases the probability of patient incompliance and discontinuance. Most other serotonin reuptake inhibitors (SRIs) have half-lives ≧24 hours. A 24-72 hour half-life is regarded as ideal for this class of compounds by most clinicians. There is therefore an obvious and immediate need for improvements in the development of monoamine reuptake inhibitors such as paroxetine.